1. Field of the Invention
The present invention relates to a heat-pipe type cooling apparatus which is used for cooling, for example, semiconductor devices.
2. Description of the Related Art
In general, a heat-pipe type cooling apparatus for cooling semiconductor devices or the like employs a plurality of heat pipes. FIG. 36 illustrates an essential part of such a type of cooling apparatus. Since each heat pipe 1 has limited capacity for transporting heat, this type of cooling apparatus is so constructed that a plurality of heat pipes are connected to a common block which has excellent heat conductivity and to which the semiconductor device to be cooled is attached. When this type of cooling apparatus is charged with water as the working fluid, such working fluid is frozen when the ambient temperature has come down below 0.degree. C. The cooling apparatus with frozen heat medium cannot perform expected function: namely, it may undesirably allow the semiconductor device to be heated beyond the maximum allowable temperature, e.g., 100.degree. C. or higher.
Japanese Patent Unexamined Publication No. 2-229455 discloses a heat-pipe type cooling apparatus in which a plurality of heat pipes are grouped into two groups which are charged with different types of heat mediums. For instance, heat pipes of a first group is charged with freon R-113 as the working fluid, while the other is charged with pure water. Freon R-113 is not frozen even when the ambient temperature has come down below 0.degree. C. and, hence, provides superior start-up characteristic even at low temperatures, as well as excellent performance at normal temperature during steady operation. Furthermore, the use of freon-type heat medium is not recommended from the view point of preservation of environment all over the earth.
A heat-pipe type cooling apparatus for cooling, for example, a semiconductor device is designed assuming the use in summer, i.e., maximum ambient air temperature of 40.degree. C. or so, such that the semiconductor temperature is maintained not to exceed a predetermined standard temperature, e.g., 100.degree. C. When the cooling apparatus is intended to be used also in winter season, the working fluid may be frozen depending on its type, when the same cooling apparatus is used also in winter season. For instance, when the ambient air temperature is as low as -30.degree. C., the heat-pipe type cooling apparatus using water as the working fluid cannot be well started up because the water is frozen at such low temperature. As the semiconductor device starts to operate, heat is generated but the transportation of heat is commenced only after the solid phase of water, i.e, ice, is molten. If the heat dissipation capacity at the condenser is large for the air temperature, the working fluid is frozen again in the condenser, with the result that the working fluid does not return to the evaporating section. Consequently, the heat-pipe type cooling apparatus fails to operate due to shortage of liquid, allowing the semiconductor device to be overheated to a temperature beyond the maximum allowable temperature.
Thus, the known heat-pipe type cooling apparatus may not function as it cannot be started up due to freezing of the working fluid. In order that the heat-pipe type cooling apparatus is started at a temperature not higher than the freezing temperature of the working fluid, it is necessary to reduce the heat dissipation effect at the condensation section of the heat pipe.